This invention relates to the utilization of superconducting quantum interference devices, and, more particularly, to the packaging in which such devices are placed for service.
A Superconducting QUantum Interference Device, commonly called by the acronym "SQUID" in the art, is a very sensitive detector of small electrical currents. The SQUID can also be used as a sensitive detector of small magnetic fields by using a magnetic flux detector such as a simple loop or a gradiometer, sometimes collectively termed an "input coil" in the art, to excite a current flow in the SQUID, and then detecting that current flow with the SQUID as a measure of the magnetic field. SQUIDs operated in conjunction with an input coil are the most sensitive magnetic field detectors presently in use. As suggested by its name, the SQUID functions in a superconducting mode of operation, and therefore is normally cooled to cryogenic temperatures of 10K or less.
The human body emits weak magnetic fields that are related to its state of health. For example, the fields produced by the brain may be on the order of 1/10,000,000 of the magnitude of the earth's magnetic field, and only a device as sensitive as the SQUID can be used to detect and investigate these biomagnetic fields. SQUIDs are therefore central to the biomagnetometer, which is a medical analytical and diagnostic instrument used to measure magnetic fields produced by the brain, heart, or other parts of the body.
A typical biomagnetometer has an input coil positioned near to the portion of the body under study, and a SQUID connected to the input coil. The SQUID is located within a container cooled to its preselected temperature of superconducting operation, and the input coil is normally located within that same container. A magnetic field emitted from the body excites a current flow in the coil, which is detected by the SQUID to produce a signal that is further processed to yield an understanding of the magnetic field produced by the body, and thence of the functioning of the body.
A SQUID is so sensitive that it must be carefully shielded from any extraneous electrical or magnetic fields that might interfere with its measurement of the emitted field from the body. The magnetic field of the earth or that produced by nearby electrical equipment, for example, can completely overwhelm and mask the biomagnetic field of interest, if the SQUID is not shielded from such extraneous field. Care is therefore taken to fully shield the SQUID from such influences.
One of the most effective shielding approaches is to place a piece of superconducting material between the SQUID and sources of extraneous magnetic and electrical fields. A superconducting material not only has a low electrical resistance, but also excludes magnetic fields through a phenomenon called the Meissner effect. Since the SQUID is operated at superconducting temperatures anyway, the use of superconducting shielding poses no additional technical problems and is very effective.
In current biomagnetometers, the SQUIDs are usually enclosed within housings made of machined niobium or other superconductor, and lead-plated machined brass. These housings are effective in shielding the SQUIDs from external magnetic and electrical fields, but are also expensive to manufacture and repair.
Accordingly, there is a need for improved shielding techniques which are effective, and also less costly than existing approaches. The present invention fulfills this need, and further provides related advantages.